Annual report pursuant to Section 13 and 15(d)

Derivative Financial Instruments - Additional Information (Detail)

v2.4.0.6
Derivative Financial Instruments - Additional Information (Detail) (USD $)
12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Derivative Instruments And Hedging Activities [Line Items]      
Unrealized gain (loss) on commodity contracts $ 6,289,000 $ (11,769,000) $ 9,511,000
Interest Rate Swap
     
Derivative Instruments And Hedging Activities [Line Items]      
Realized gain (loss) on interest derivatives     (4,700,000)
Unrealized gain (loss) on commodity contracts     $ 4,400,000
Derivative fixed interest rate     5.21%
Derivative contract expiration date     Aug. 01, 2010